Red squirrel in a nest of lichen and pine needles, Scottish Highlands

Snug as a squirrel in its nest. Red squirrel

Shhh, it's sleeping. But don't confuse that with hibernating. A tree squirrel, like this red squirrel in Scotland, doesn't hibernate during the winter. Hibernating is more of a ground squirrel trait. But the red squirrel is particular about where it sleeps, mostly building its nest, or drey, high above the ground in the cavity of an evergreen tree. Nests are often lined with soft materials such as leaves, moss, and grass. This little sleeper is using its bushy tail to keep warm, with the help of some lichen and pine needles.

In Scotland and elsewhere in the United Kingdom, the native red squirrel is threatened by the much more prominent, non-native North American gray squirrel. The American transplants tend to outcompete the red squirrels for food and living space. While there are millions of gray squirrels in the UK, it's estimated only 120,000 to 160,000 red squirrels remain, about three-quarters of them in Scotland. Conservation efforts focus on preserving red squirrel habitat and combating the spread of gray squirrels.